1. Field of the Invention
This invention relates to a scanning optical apparatus for forming a pattern onto a scanning surface, and particularly to an apparatus where a high resolution pattern is required, such as a laser make-up device (i.e., a plate making device).
2. Description of the Prior Art
In order to draw a high accuracy pattern, it is necessary that a laser beam is focused into a spot as small as possible. In order to make the spot small, an F number of the optical system is required to be small. As a result, a focal depth becomes shallow. Therefore, a curvature of field must be reduced so that the predetermined spot diameter can be maintained in every point of the scanning surface.
However, in the case of general scanning optical systems such as a laser beam printer, since the laser beam makes incident to the polygon mirror from out of the optical axis of the scanning lens, a deflecting point on the reflecting surface of the polygon mirror changes asymmetrically about the optical axis. Therefore, there is a problem that the curvature of field becomes asymmetric on the scanning surface and correction can not be made when a symmetric lens is used about the optical axis.
In order to reduce the curvature of field, it is effective to make the change of the deflecting point symmetric with respect to the optical axis.
To solve the above problem, a laser beam can be made incident to a polygon mirror through the optical axis of the f.theta. lens. According to this construction, since the appearance of the curvature of field is made symmetric, correction can be made using a symmetric lens with respect to the optical axis and the value becomes very small.
However, in order to employ such a construction, it is necessary that a static deflector for separating a beam directed to the polygon mirror from a beam incoming to an f.theta. lens is disposed in the optical path.
Japanese Patent Early Laid-open Publication No. Sho 60-233616 discloses a construction in which a light beam from a light source is made incident to a polygon mirror through the optical axis of a scanning lens using a polarizing beam splitter and a 1/4 wave plate.
It is noted that in the construction of Japanese Patent Early Laid-open Publication No. Sho 60-233616, a laser beam is temporarily imaged on a reflecting surface of the polygon mirror in an auxiliary scanning plane in order to correct adverse effects caused by a tilting error of the reflecting surface of the polygon mirror with respect to a rotational axis thereof.
However, according to the construction of the above-mentioned Publication, because the angle of incidence of the light beam to a polarized beam splitter is changed in accordance with the rotation of the polygon mirror, the transmittance of the polarized beam splitter is gradually changed as it goes toward the peripheral portion from the central portion and irregularity of light quantity occurs on the scanning surface.